Method for producing organic EL element, display device, light-emitting apparatus, and ultraviolet irradiation device

ABSTRACT

A method of manufacturing an organic EL element operating at low voltage to emit light at high intensity comprises: a first step of forming, on an anode, a hole injection layer including metal oxide; a second step of irradiating the hole injection layer with ultraviolet light, the ultraviolet light having a wavelength greater than a wavelength at which oxygen molecules decompose and yield oxygen radicals; a third step of forming functional layers containing organic material on or above the hole injection layer after the second step, the functional layers including a light-emitting layer; and a fourth step of forming a cathode on or above the functional layers.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation application of PCT Application No.PCT/JP2010/004216 filed Jun. 24, 2010, designating the United States ofAmerica, the disclosure of which, including the specification, drawingsand claims, is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to methods of manufacturing an organicelectric-field light-emitting element (hereinafter referred to as an“organic EL element”), which is an electric light emitter. The presentdisclosure also relates to display apparatuses and light-emittingapparatuses that are manufactured by the methods, and to ultravioletirradiation apparatuses used in the methods. The present disclosurerelates in particular to a technology for cleaning the surface of a holeinjection layer.

DESCRIPTION OF THE RELATED ART

In recent years, progress is being made in research and development ofdiverse functional elements which involve the use of an organicsemiconductor. One typical example of a functional element is an organicEL element. An organic EL element is a current-driven light emitter, andcommonly has a pair of electrodes, namely an anode and a cathode, andfunctional layers layered between the pair of electrodes. The functionallayers include a light-emitting layer composed of an organic material.Upon application of voltage across the pair of electrodes, holesinjected from the anode to the functional layers recombine withelectrons injected from the cathode to the functional layers. Therecombination causes the phenomenon of electroluminescence, whichinvolves emission of light. Being self-luminescent, an organic ELelement is highly visible. In addition, being completely solid, anorganic EL element has excellent impact resistance. Owing to theseadvantages, more attention is being given to the applications of organicEL elements as a light emitter or a light source for various displayapparatuses.

To cause an organic EL element to emit light at high intensity,efficient injection of carriers (i.e., holes and electrons) from theelectrodes to the functional layers is important. Generally, theprovision of injection layers between each of the electrodes and afunctional layer is effective in facilitating efficient injection ofcarriers. This is because an injection layer serves to lower the energybarrier to be overcome in the injection of carriers. An injection layerdisposed between a functional layer and the anode is a hole-injectionlayer composed of an organic material, such as copper phthalocyanine orPEDOT (conductive polymer), or of a metal oxide, such as molybdenumoxide or tungsten oxide. An injection layer disposed between afunctional layer and the cathode is an electron injection layer composedof an organic material, such as metal complex or oxadiazole, or of ametal, such as barium.

It has been reported that organic EL elements having a hole injectionlayer composed of a metal oxide, such as molybdenum oxide or tungstenoxide, exhibit improved hole injection efficiency and longevity (seePatent Literature 1 and Non-Patent Literature 1). It is further reportedthat the improvement achieved is relevant to the energy level resultingfrom structures similar to oxygen vacancies of metal oxide on thesurface of the hole injection layer (see Non-Patent Literature 2).

CITATION LIST Patent Literature

-   [Patent Literature 1]

Japanese Patent Application Publication No. 2005-203339

-   [Non-Patent Literature]-   [Non-Patent Literature 1]

Jingze Li et al., Synthetic Metals 151, 141 (2005).

-   [Non-Patent Literature 2]

Kaname Kanai et al., Organic Electronics 11, 188 (2010).

-   [Non-Patent Literature 3]

J. B. Pedley et al., Journal of Physical and Chemical Reference Data 12,967 (1984).

-   [Non-Patent Literature 4]

I. N. Yakovkin et al., Surface Science 601,1481 (2007).

SUMMARY

In the manufacturing of an organic EL element, problems are presented byadsorbates, mainly carbon-containing adsorbates, derived from moleculesof carbon dioxide, water, and organic material contained in theatmosphere or from molecules of impurities generated during themanufacturing steps. To be more specific, in a step of laminating therespective layers, such as electrodes and a hole injection layer, of anorganic EL element, if a layer with adsorbates on its surface is toppedwith another layer, the adsorbates are embedded between the layers. Thepresence of adsorbates involves the risk of increasing the drive voltageof, and/or reducing the longevity of the resulting organic EL element.

One non-limiting and exemplary embodiment provides a method ofmanufacturing an organic EL element operating at low voltage to emitlight at high intensity.

Solution to Problem

In one general aspect, the techniques disclosed here feature a method ofmanufacturing an organic EL element comprising: a first step of forming,on an anode, a hole injection layer including metal oxide; a second stepof irradiating the hole injection layer with ultraviolet light, theultraviolet light having a wavelength greater than a wavelength at whichoxygen molecules decompose and yield oxygen radicals; a third step offorming functional layers containing organic material on or above thehole injection layer after the second step, the functional layersincluding a light-emitting layer; and a fourth step of forming a cathodeon or above the functional layers.

Advantageous Effects of Invention

According to the method of manufacturing an organic EL elementpertaining to an aspect of the present disclosure, after the formationof a hole injection layer including metal oxide, a surface thereof isexposed to ultraviolet light of a predetermined wavelength longer than awavelength that causes decomposition of oxygen molecules to yield oxygenradicals, and accordingly the adsorbates attached to the surface of thehole injection layer can be removed without making the energy levelresulting from structures similar to oxygen vacancies of metal oxide onthe surface of the hole injection layer disappear. Therefore, it ispossible to manufacture an organic EL element operating at low voltageto emit light at high intensity.

These general and specific aspects may be implemented using a device.

Additional benefits and advantages of the disclosed embodiments will beapparent from the specification and figures. The benefits and/oradvantages may be individually provided by the various embodiments andfeatures of the specification and drawings disclosure, and need not allbe provided in order to obtain one or more of the same.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a structure ofan organic EL element pertaining to an embodiment of the presentinvention.

FIG. 2 is a view illustrating an overall structure of a displayapparatus pertaining to one aspect of the present invention.

FIG. 3A is a longitudinal sectional view illustrating a light-emittingapparatus pertaining to one aspect of the present invention, and FIG. 3Bis a transverse sectional view illustrating the light-emittingapparatus.

FIGS. 4A, 4B, and 4C are views illustrating important parts of a methodfor manufacturing the organic EL element according to the embodiment.

FIG. 5 is a view illustrating UPS spectra of tungsten oxide.

FIG. 6 is a view illustrating UPS spectra of tungsten oxide.

FIG. 7 is a view illustrating XPS spectra of tungsten oxide.

FIG. 8 is a view illustrating UPS spectra of tungsten oxide.

FIG. 9 is a view illustrating XPS spectra of tungsten oxide.

FIG. 10 is a schematic cross-sectional view illustrating a structure ofa hole-only device.

FIG. 11 is a device characteristics diagram of relation curves eachillustrating a relation between applied voltage and electric currentdensity of a different hole-only device.

FIG. 12 is a device characteristics diagram of relation curves eachillustrating a relation between applied voltage and electric currentdensity of a different organic EL element.

FIG. 13 is a view illustrating the spectral distribution of a metalhalide lamp employed in an embodiment of the present invention.

FIG. 14 is a view illustrating the surface configuration of tungstenoxide.

FIG. 15 is a view illustrating XPS spectra of molybdenum oxide.

FIG. 16 is a view illustrating UPS spectra of molybdenum oxide.

FIG. 17 is a view illustrating XPS spectra of molybdenum oxide.

DETAILED DESCRIPTION

[Outline of Aspects of the Present Invention]

An aspect of the present invention provides a method of manufacturing anorganic EL element comprising: a first step of forming, on an anode, ahole injection layer including metal oxide; a second step of irradiatingthe hole injection layer with ultraviolet light, the ultraviolet lighthaving a wavelength greater than a wavelength at which oxygen moleculesdecompose and yield oxygen radicals; a third step of forming functionallayers containing organic material on or above the hole injection layerafter the second step, the functional layers including a light-emittinglayer; and a fourth step of forming a cathode on or above the functionallayers. Therefore, oxygen molecules are unlikely to decompose and yieldoxygen radicals, and the energy level resulting from structures similarto oxygen vacancies of metal oxide, which affects the hole injectionefficiency, is unlikely to disappear.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, in the second step, theultraviolet light has a wavelength greater than a wavelength at whichozone decomposes and yields oxygen radicals. In this case, ozonemolecules are unlikely to decompose and yield oxygen radicals, and theenergy level resulting from structures similar to oxygen vacancies ofmetal oxide is unlikely to disappear.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, in the second step, theultraviolet light has a wavelength longer than 184.9 nm and not longerthan 380 nm as a main range. In the atmosphere or a gas atmospherecontaining oxygen molecules, a wavelength of ultraviolet light at whichoxygen molecules decompose and yield oxygen radicals that are thencombined with remaining oxygen molecules to generate ozone is 184.9 nm.Accordingly, when ultraviolet light having a wavelength longer than184.9 nm is used, the oxygen molecules are unlikely to decompose andyield oxygen radicals, and the energy level resulting from structuressimilar to oxygen vacancies of metal oxide is unlikely to disappear.

Note that in the spectrum of the light, the integrated intensitycorresponding to the ultraviolet light having a wavelength longer than184.9 nm and not longer than 380 nm as a main range occupies 98% of theintegrated intensity corresponding to the range of ultraviolet light notlonger than 380 nm.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, in the second step, theultraviolet light has a wavelength longer than 253.7 nm and not longerthan 380 nm as a main range. The wavelength of ultraviolet light atwhich ozone decomposes and yields oxygen radicals again is 253.7 nm.Accordingly, when ultraviolet light having a wavelength longer than253.7 nm is used, the oxygen molecules are more unlikely to decomposeand yield oxygen radicals, and the energy level resulting fromstructures similar to oxygen vacancies of metal oxide is more unlikelyto disappear.

Note that ultraviolet light having a wavelength longer than 253.7 nm andnot longer than 380 nm as a main range means as follows: in the spectrumof the light, the integrated intensity corresponding to the range ofultraviolet light longer than 253.7 nm and not longer than 380 nmoccupies 80% of the integrated intensity corresponding to the range ofultraviolet light not longer than 380 nm.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, the first step isperformed in vacuum, and the second step is performed in the atmosphere.By performing the first step in vacuum, it is possible to form, under apredetermined condition for forming film, a hole injection layer whosesurface has the energy level resulting from structures similar to oxygenvacancies of metal oxide. By performing the second step in theatmosphere, application to large panels is easy.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, in the second step, thehole injection layer is irradiated with the ultraviolet light until anarrow-scan spectrum for an inner-shell orbital of an atom included as amain element in the metal oxide stabilizes in XPS measurement. In thisphase, the adsorbate removal effect is assumed to have reached a levelof saturation, so that a sufficient adsorbate removal effect is expectedto be achieved.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, in the first step, themetal oxide is tungsten oxide.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, in the second step, theultraviolet light has a wavelength corresponding to an energy valueequal to or greater than binding energy of a single bond between anoxygen atom in tungsten oxide and adsorbates to the oxygen atom, andsmaller than binding energy between an oxygen atom and a tungsten atomin tungsten oxide. In this case, adsorbates are removed without breakingatomic bonds among oxygen atoms and tungsten atoms. That is, adsorbatesare removed by disconnecting atomic bonds appearing in adsorbateswithout making tungsten oxide chemically active, i.e., while tungstenoxide is chemically stable.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, the adsorbates includeat least one of carbon atom, hydrogen atom, oxygen atom and nitrogenatom. Further, in a specific aspect of the present invention directed tothe method of manufacturing the organic EL element, in the second step,the hole injection layer is exposed to ultraviolet light of a wavelengthgreater than a wavelength at which oxygen molecules decompose and yieldoxygen radicals. In this case, oxygen molecules are unlikely todecompose and yield oxygen radicals, and the energy level resulting fromstructures similar to oxygen vacancies of metal oxide is unlikely todisappear.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, the metal oxide istungsten oxide, and in the second step, the hole injection layer isirradiated with the ultraviolet light until UPS spectrum stabilizeswithin the binding energy range from 4.5 eV to 5.4 eV. In this phase,the adsorbate removal effect is assumed to have reached a level ofsaturation, so that a sufficient adsorbate removal effect is expected tobe achieved.

Note that a numerical range stated as “from . . . to . . . ” is intendedto mean that the upper and lower limits are both inclusive. For example,a range from 4.5 eV to 5.4 eV includes 4.5 eV and 5.4 eV.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, the metal oxide ismolybdenum oxide, and in the second step, the hole injection layer isirradiated with the ultraviolet light until UPS spectrum stabilizeswithin the binding energy range from 3.7 eV to 5.2 eV. In this phase,the adsorbate removal effect is assumed to have reached a level ofsaturation, so that a sufficient adsorbate removal effect is expected tobe achieved.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, in the second step, thehole injection layer is irradiated with the ultraviolet light until anarrow-scan spectrum for C1s of the hole injection layer stabilizes inXPS measurement. In this phase, the adsorbate removal effect is assumedto have reached a level of saturation, so that a sufficient adsorbateremoval effect is expected to be achieved.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, the metal oxide istungsten oxide, and in the second step, the hole injection layer isirradiated with the ultraviolet light until a narrow-scan spectrum forW4f of the hole injection layer stabilizes in XPS measurement. In thisphase, the adsorbate removal effect is assumed to have reached a levelof saturation, so that a sufficient adsorbate removal effect is expectedto be achieved.

Further, in a specific aspect of the present invention directed to themethod of manufacturing the organic EL element, the metal oxide ismolybdenum oxide, and in the second step, the hole injection layer isirradiated with the ultraviolet light until a narrow-scan spectrum forMo3d of the hole injection layer stabilizes in XPS measurement. In thisphase, the adsorbate removal effect is assumed to have reached a levelof saturation, so that a sufficient adsorbate removal effect is expectedto be achieved.

A display apparatus pertaining to an aspect of the present disclosureinvolves the use of the organic EL element manufactured by any of theabove manufacturing methods. Therefore, the organic EL element operatesat low voltage to emit light at high intensity, and the performance ofthe display apparatus is high.

A light-emitting apparatus pertaining to an aspect of the presentdisclosure involves the use of the organic EL element manufactured byany of the above manufacturing methods. Therefore, the organic ELelement operates at low voltage to emit light at high intensity, and theperformance of the display apparatus is high.

An ultraviolet irradiation apparatus pertaining to the presentdisclosure is an ultraviolet irradiation apparatus irradiatingultraviolet light on an intermediate product of an organic EL elementhaving a hole injection layer and functional layers layered between ananode and a cathode, the hole injection layer including metal oxide, thefunctional layers including organic material and having holes injectedthereto from the hole injection layer, wherein the ultraviolet light hasa wavelength greater than a wavelength at which oxygen moleculesdecompose and yield oxygen radicals. Therefore, oxygen molecules areunlikely to decompose and yield oxygen radicals, and the adsorbatesattached to the surface of the hole injection layer can be removedwithout making the energy level resulting from structures similar tooxygen vacancies of metal oxide on the surface of the hole injectionlayer disappear.

Further, in a specific aspect of the present invention directed to theultraviolet irradiation apparatus, the ultraviolet light has awavelength greater than a wavelength at which ozone decomposes andyields oxygen radicals. In this case, ozone molecules are unlikely todecompose and yield oxygen radicals, and the energy level resulting fromstructures similar to oxygen vacancies of metal oxide is unlikely todisappear.

Further, in a specific aspect of the present invention directed to theultraviolet irradiation apparatus, the ultraviolet light has awavelength longer than 184.9 nm and not longer than 380 nm as a mainrange. In this case, ultraviolet light having a wavelength longer than184.9 nm and not longer than 380 nm as a main range is emitted.Accordingly, oxygen radicals are unlikely to be yielded, and theadsorbates attached to the surface of the hole injection layer can beremoved without affecting the energy level resulting from the structuressimilar to oxygen vacancies of metal oxide.

In a specific aspect of the present invention directed to theultraviolet irradiation apparatus, the ultraviolet light has awavelength longer than 253.7 nm and not longer than 380 nm as a mainrange. In this case, oxygen radicals are more unlikely to be yielded,and the energy level resulting from structures similar to oxygenvacancies of metal oxide is more unlikely to disappear.

[Developments Leading to the Present Invention]

With the aim of preventing increase in drive voltage of the organic ELelement and reduction in longevity of the organic EL element, thepresent inventors have come to a technical idea of adding, to themanufacturing process, a cleaning step of removing adsorbates from thelayer surfaces subsequently to the formation of the respective layers.

As the cleaning methods for removing adsorbates, the present inventorshave turned their attention to ultraviolet (UV) ozone cleaning andoxygen plasma cleaning, which are widely used for cleaning glasssubstrates and electrodes, for the high degree of cleanliness achievedthereby.

Due to the high degree of cleanliness of these cleaning methods, theinventors initially estimated that these methods could be simply appliedto the process for removing adsorbates attached to each layer of theorganic EL element.

In practice, however, intensive studies by the present inventors on thecleaning methods have revealed that neither UV ozone cleaning nor oxygenplasma cleaning is suitable for cleaning the hole injection layer of anorganic EL element, provided that the hole injection layer is composedof a metal oxide, such as molybdenum oxide or tungsten oxide.

The reason is as follows. Both the UV ozone cleaning and oxygen plasmacleaning utilize strong oxidation associated with oxygen radicals formedby decomposition of oxygen molecules. Through the oxidation, oxygenatoms end up filling structures similar to oxygen vacancies of metaloxide on the surface of the hole injection layer. Consequently, theenergy level resulting from structures similar to oxygen vacanciesdisappears from the hole injection layer composed of metal oxide. As aresult, there is a risk of decreasing the hole injection efficiency. Tobe more precise, the present inventors have confirmed, by experimentsdescribed below, that the energy level resulting from structures similarto oxygen vacancies almost completely disappear through UV ozonecleaning.

Due to the knowledge obtained above, the present inventors haverecognized that in a process for removing adsorbates attached to thehole injection layer, which is made from metal oxide, of the organic ELelement, it is important to prevent beforehand decomposition of oxygenmolecules to yield oxygen radicals. As a result of this, it is possibleto prevent a decrease of efficiency of hole injection, while removingthe adsorbates.

As a result, the inventors arrived at a feature of the present inventionof exposing the hole injection layer made of metal oxide to ultravioletlight having a wavelength longer than a wavelength that causesdecomposition of oxygen molecules to yield oxygen radicals.

After a series of researches and studies regarding the above features ofthe present invention, the present inventors came to be aware ofNon-Patent Literature 1 disclosing UV ozone cleaning performedsubsequently to the formation of a hole injection layer composed oftungsten oxide. However, Non-Patent Literature 1 discloses nothing aboutthe impact on the characteristics of the organic EL element imposed byUV ozone cleaning and nothing about optimizing the conditions of UVozone cleaning. It is further noted that Non-Patent Literature 1 doesnot describe anything about the fact, which are found by the presentinventors through specific studies, that UV ozone cleaning is notsuitable for cleaning a hole injection layer composed of tungsten oxideunless adequately modified. Naturally, Non-Patent Literature 1 does notdescribe anything about the technical reasons for the unsuitability.

As another method for removing adsorbates, the following discussessputter etching of performing argon ion sputtering in vacuum chambersubsequently to a hole injection layer is formed. The sputter etchinghas been reported to remove adsorbates and also to increase the energylevel resulting from structures similar to oxygen vacancies. Thus, thesputter etching appears to be an excellent cleaning method at first.

Unfortunately, the clean surface and the increased energy level obtainedby the sputter etching can be maintained only in the vacuum chamber forthe following reason. That is, the surface of a hole injection layertreated by sputter etching in vacuum is highly instable because themolecular bonds have been forcedly broken by an ion beam. Therefore,once taken out of the vacuum chamber, the hole injection layer easilyadsorbs atmospheric molecules to be stabilized. In the manner describedabove, structures similar to oxygen vacancies of metal oxide which arecreated in vacuum are instantly filled, and the layer surface oncecleaned rapidly adsorbs contaminants.

The layer surface may be kept clean by performing some or all of thesubsequent manufacturing steps continuously inside the vacuum chamber.However, performing manufacturing steps inside a vacuum chamber isapplicable, on condition that the organic EL panel to be manufactured isrelatively small. For a large-sized organic EL panel having display sizeof e.g., around 50 inches, it is extremely difficult to perform themanufacturing steps inside a vacuum chamber as the vacuum chamber needsto be large enough for such a large-sized organic EL panel. Besides, thethroughput of steps performed inside a vacuum chamber is small, and suchsteps are not desirable for mass production.

Alternatively to removing adsorbates, a method of preventing adhesion ofcontaminants per se is one possibility. For example, by performing someor all of manufacturing steps subsequent to the layer formationcontinuously inside a vacuum chamber, the respective layers are exposedneither to the atmosphere nor to impurity molecules. Thus, the layersurface is kept free or substantially free of contaminants. However,this scheme is extremely difficult to apply to the manufacturing oflarge-sized organic EL panels because the vacuum chamber of acorresponding size is required as already described above.

Alternatively, performing manufacturing steps inside a chamber filledwith inert gas is another possibility. This scheme is applicable to themanufacturing of large-sized organic EL panels. Unfortunately, such achamber still contains impurity molecules and the like, although theamount is smaller than that in the atmosphere. In addition, completeremoval of such molecules from the chamber is difficult.

As described above, it is extremely difficult to remove adsorbatesattached to the surface of the hole injection layer without eliminatingthe energy level resulting from structures similar to oxygen vacanciesof metal oxide on the surface of the hole injection layer and withoutusing a vacuum chamber. In contrast, the method of manufacturing theorganic EL element pertaining to one aspect of the present inventionsolves this problem by, after the formation of a hole injection layerincluding metal oxide, exposing the hole injection layer to ultravioletlight of a wavelength greater than a wavelength that causesdecomposition of oxygen molecules to yield oxygen radicals.

According to a manufacturing method of an organic EL element pertainingto one aspect of the present invention, an energy level resulting fromstructures similar to oxygen vacancies of metal oxide on the surface ofthe hole injection layer is maintained without being eliminated.Therefore, holes are injected from the anode to the functional layerswith efficiency. Consequently, the organic EL element emits light at lowpower consumption and high intensity.

Further, according to a manufacturing method of an organic EL elementpertaining to one aspect of the present invention, adsorbates attachedto the surface of the hole injection layer are removed and theadsorbates are not embedded between the hole injection layer and thefunctional layers. As a consequence, the drive voltage of the organic ELelement is not increased and carrier traps decreasing a life of theorganic EL element, such as impurities derived from adsorbates, are notformed, which ensures favorable characteristics of the organic ELelement.

Further, a manufacturing method of an organic EL element pertaining toone aspect of the present invention can be performed in the atmosphere.Accordingly, this method can be easily applied to large organic ELpanels, and is desirable for mass production.

Further, in a manufacturing method of an organic EL element pertainingto one aspect of the present invention, an energy level resulting fromstructures similar to oxygen vacancies of metal oxide on the surface ofthe hole injection layer is continuously maintained throughout the timefrom completion of the cleaning of the surface of the hole injectionlayer to the formation of upper layers. Consequently, the ability ofhole injection does not decrease. This ensures the stable manufacturingof the organic EL element that is driven with low drive voltage and haslongevity.

In addition, the duration and intensity of ultraviolet light are set sothat, with respect to a photoelectron spectrum exhibited by the holeinjection layer, changes in shape of a spectral region corresponding toa predetermined range of binding energy converge. With the setting, theirradiation condition can be clearly determined to remove adsorbates toa maximum, and the highly stable hole injection efficiency is realizedwith a minimum cleaning process.

Further, removal of the adsorbates by irradiating ultraviolet lightdescribed above may be performed in the atmosphere, in addition to invacuum or inert gas atmosphere. This scheme is therefore applicable tothe manufacturing of large-sized organic EL panels.

[Embodiments]

In order to explain a manufacturing method of an organic EL elementpertaining to one aspect of the present invention, the following firstdescribes an organic EL element, a display apparatus, and alight-emitting apparatus all manufactured by the manufacturing method.Next, the manufacturing method is described. Subsequently, the resultsof experiments conducted to confirm the performance of the organic ELelement are described, followed by observations on the experimentalresults. In addition, ultraviolet irradiation apparatus pertaining toone aspect of the present invention is described. Note that each figureis illustrated on a reduced scale different from the proportion of theactual sizes.

<Structure of Organic EL Element>

FIG. 1 is a schematic cross-sectional view illustrating the structure ofan organic EL element 1 pertaining to an embodiment of the presentinvention.

The organic EL element 1 is, for example, an application-type organic ELelement, which has functional layers formed by applying raw material bya wet process. The organic EL element 1 includes a hole injection layer3, various functional layers (a buffer layer 4 and a light-emittinglayer 5, in this embodiment) each containing a functional materialhaving a predetermined function, and a pair of electrodes, namely ananode 2 and a cathode 6. The hole injection layer 3 and the functionallayers are layered between the pair of electrodes.

More specifically, the organic EL element 1 includes, as illustrated inFIG. 1, the anode 2, the hole injection layer 3, the buffer layer 4, thelight-emitting layer 5, and the cathode 6 (composed of a barium layer 6a and an aluminum layer 6 b), which are layered in the stated order onone main surface of a substrate 7.

(Hole Injection Layer)

In one example, the hole injection layer 3 is a thin film (layer) havinga thickness of 30 nm and composed of tungsten oxide, which is a metaloxide. In the formula (WOx) representing the composition of tungstenoxide, x is a real number generally falling within the range of 2<x<3.It is preferable that the hole injection layer 3 consist only oftungsten oxide. However, the inclusion of a trace level of impurities isacceptable, provided that the amount does not exceed the amount ofimpurities which may be incorporated as a result of normal processing.

By being formed under the predetermined conditions, the resulting holeinjection layer 3 is ensured to have, on its surface, the energy levelresulting from structures similar to oxygen vacancies of the metaloxide. The presence of the energy level enables effective holeinjections. In addition, subsequently to the layer formation, the holeinjection layer 3 is irradiated with ultraviolet light of apredetermined wavelength in the atmosphere. As a result of theultraviolet (UV) irradiation, the surface of the hole injection layer 3is cleaned to reduce adsorbates thereon, without affecting the energylevel resulting from the structures similar to oxygen vacancies of metaloxide. In addition, the duration and intensity of UV irradiation is setso that, with respect to a photoelectron spectrum exhibited by the holeinjection layer 3 after the UV irradiation, changes in shape of aspectral region corresponding to a predetermined range of binding energyconverge. With the setting, the maximum effect of removing adsorbates isachieved by UV irradiation performed at the minimum conditions.

(Buffer Layer)

In one example, the buffer layer 4 is a 20 nm-thick layer of TFB(poly(9,9-di-n-octylfluorene-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)),which is an amine-containing organic polymer.

(Light-Emitting Layer)

In one example, the light-emitting layer 5 is a 70 nm-thick layer ofF8BT (poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)), which is anorganic polymer. However, the material of the light-emitting layer 5 isnot limited to this, and the light-emitting layer 5 may contain acommonly-known organic material. Examples of such commonly-known organicmaterial for the light-emitting layers 5 include fluorescent material,such as an oxinoid compound, perylene compound, coumarin compound,azacoumarin compound, oxazole compound, oxadiazole compound, perinonecompound, pyrrolo-pyrrole compound, naphthalene compound, anthracenecompound, fluorene compound, fluoranthene compound, tetracene compound,pyrene compound, coronene compound, quinolone compound and azaquinolonecompound, pyrazoline derivative and pyrazolone derivative, rhodaminecompound, chrysene compound, phenanthrene compound, cyclopentadienecompound, stilbene compound, diphenylquinone compound, styryl compound,butadiene compound, dicyanomethylene pyran compound, dicyanomethylenethiopyran compound, fluorescein compound, pyrylium compound,thiapyrylium compound, selenapyrylium compound, telluropyryliumcompound, aromatic aldadiene compound, oligophenylene compound,thioxanthene compound, anthracene compound, cyanine compound, acridinecompound, metal complex of an 8-hydroxyquinoline compound, metal complexof a 2-bipyridine compound, complex of a Schiff base and a group threemetal, metal complex of oxine, rare earth metal complex, etc., asrecited in Japanese Patent Application Publication No. H5-163488.

(Functional Layers)

A functional layer according to the present invention refers to one of:a hole transport layer that transfers holes; a light-emitting layer thatemits light as a result of recombination of injected holes andelectrons; and a buffer layer used for adjusting optical characteristicsof the organic EL element 1 or for blocking electrons. Alternatively,functional layers according to the present invention may refer to acombination of two or more of, or all of the above-mentioned layers.Although the present invention is directed to a hole injection layer, anorganic EL element commonly includes layers having the functions of thehole transport layer, light-emitting layer and the like described above,in addition to the hole injection layer. As such, the expression“functional layers” refers to all such layers which need to be includedin the organic EL element, aside from the hole injection layer to whichthe present invention is directed.

(Other)

In one example, the anode 2 is a thin ITO film having a thickness of 50nm. In one example, the cathode 6 includes the barium layer 6 a of 5nm-thick and the aluminum layer 6 b of 100 nm-thick, which are layeredone on top of the other. The anode 2 and the cathode 6 are connected toa direct current (DC) voltage source 8 to supply power to the organic ELelement 1 from the outside.

In one example, the substrate 7 may be formed with one of insulatingmaterials, such as alkali-free glass, soda glass, nonfluorescent glass,phosphate glass, borate glass, quartz, acrylic resin, styrenic resin,polycarbonate resin, epoxy resin, polyethylene, polyester, siliconeresin, and alumina.

(Effects and Advantages Produced by the Organic EL Element)

As described above, the hole injection layer 3 of the organic EL element1 contains tungsten oxide, which is a metal oxide. In addition, afterthe formation of the hole injection layer 3, a surface thereof isexposed to ultraviolet light of a predetermined wavelength. As a result,adsorbates on the surface of the holes injection layer 3 have beenremoved to a maximum, without affecting the energy level resulting fromstructures similar to oxygen vacancies present in metal oxide. Thisenables the organic EL element to be driven with low drive voltage andhave longevity.

<Structure of Display Apparatus>

With reference to FIG. 2, the following describes a display apparatusaccording to one aspect of the present invention. FIG. 2 is a viewillustrating an overall structure of the display apparatus according tothe aspect of the present invention.

As illustrated in FIG. 2, the display apparatus 100 includes a displaypanel 110 having organic EL elements manufactured by the methodaccording to one aspect of the present invention and also includes adrive control unit 120 connected to the display panel 100. The displayapparatus 100 may be used, for example, in a display, television, andmobile phone. The drive control unit 120 includes four drive circuits121-124 and a control circuit 125. However, in an actual displayapparatus 100, the arrangement and connection of the drive control unit120 with respect to the display panel 110 is not limited to as describedabove.

The display apparatus 100 having the above structure is excellent in itsimage quality owing to the excellent light-emitting characteristics ofthe organic EL elements.

<Structure of Light-Emitting Apparatus>

FIG. 3 illustrate a light-emitting apparatus according to one aspect ofthe present invention. More specifically, FIG. 3A is a longitudinalsectional view, whereas FIG. 3B is a transverse sectional view. As shownin FIGS. 3A and 3B, the light-emitting apparatus 200 includes: organicEL elements 210 manufactured by the method according to the aspect ofthe present invention; a base 220 having the organic EL elements 210mounted on its upper surface; and a pair of reflecting members 230disposed to flank an array of the organic EL elements 210. Thelight-emitting apparatus 200 may be used as an illuminator and a lightsource. The organic EL elements 210 are electrically connected to aconductive pattern (not illustrated) formed on the base 220 and emitlight on power supplied via the conductive pattern. Part of lightemitted from the organic EL elements 210 is reflected by the reflectingmembers 230, whereby the light distribution is controlled.

The light-emitting apparatus 200 having the above structure is excellentin its image quality owing to the excellent light-emittingcharacteristics of the organic EL elements.

(Manufacturing Method of Organic EL Elements)

The following describes a method for manufacturing the organic ELelement 1 with reference to FIG. 4. FIGS. 4A, 4B, and 4C are viewsillustrating important parts of the method for manufacturing an organicEL element according to the embodiment.

Firstly, the substrate 7 is placed inside the chamber of a sputteringfilm-forming apparatus. Then, a predetermined sputtering gas isintroduced into the chamber to form the 50 nm-thick anode 2 composed ofITO by reactive sputtering.

Next, the hole injection layer 3 is formed also by reactive sputtering.More specifically, after replacing the target with metal tungsten,reactive sputtering is performed. An argon gas and an oxygen gas areintroduced into the chamber as the sputtering gas and the reactive gas,respectively. Under this state, high voltage is applied to ionize theargon, so that the ionized argon is caused to bombard the sputteringtarget. The metal tungsten thus sputtered reacts with the oxygen gas toform tungsten oxide, which is then deposited on the anode 2 previouslyformed on the substrate 7. As a result, the hole injection layer 3 isfoamed. FIG. 4A shows an intermediate product 9 obtained at this stageof manufacturing.

The film formation described above is performed under the followingconditions: (i) the substrate temperature is not controlled; (ii) gaspressure (total gas pressure) is equal to 2.3 Pa; (iii) the ratio ofoxygen partial pressure to the total gas pressure is equal to 50%; and(iv) input power per unit surface area of the sputtering target (inputpower density) is equal to 1.2 W/cm². The hole injection layer 3composed of tungsten oxide formed under the above conditions has theenergy level resulting from structures similar to oxygen vacancies onits surface.

Subsequently to the layer formation, the substrate 7 is taken out of thechamber to the atmosphere. At this point, the hole injection layer 3 isexposed to the atmosphere and thus adsorbs gas molecules on its exposedsurface. It is also assumed that once the hole injection layer 3 isformed, impurity molecules present in the chamber adhere to the layersurface even before the substrate 7 is taken out of the chamber.

Next, as illustrated in FIG. 4B, the step of UV irradiation is performedin the atmosphere to expose the surface of the hole injection layer 3 toultraviolet light. In this step, an ultraviolet (UV) irradiationapparatus 20 according to one aspect of the present invention is used.The UV irradiation apparatus 20 has a metal halide lamp manufactured byUshio Inc. (Model No.: UVL-3000M2-N) as a light source 21. The UVirradiation apparatus 20 will be described later in detail. Theirradiation conditions are separately determined by experimentsconducted using photoemission spectroscopy measurements, which will bedescribed later. Specifically, the irradiation conditions are determinedso that changes in shape of the resulting photoelectron spectrum in aspectral region corresponding to a predetermined binding energy rangeconverge. In this embodiment, the irradiation intensity is determined tobe 155 mW/cm² and the irradiation duration is determined to be 10minutes.

Next, an ink composition containing an organic amine-containingmolecular material is dripped onto the surface of the hole injectionlayer 3 by a wet process, such as a spin coat method or inkjet method,followed by removal of the solvent of the ink composition byvolatilization. As a result, the buffer layer 4 is formed, and thus anintermediate product 10 as illustrated in FIG. 4C is obtained.

Next, an ink composition containing an organic light-emitting materialis dripped onto the surface of the buffer layer 4 by a similar method asin the forming of the hole injection layer 3, followed by removal of thesolvent of the ink composition by volatilization. Thus, thelight-emitting layer 5 is formed.

Note that the method employed to form the buffer layer 4 and thelight-emitting layer 5 is not limited to the specific method describedabove. Alternatively, the ink may be dripped or applied by any othermethod than the spin coat method or inkjet method. Examples of suchalternative methods include a dispenser method, nozzle coating method,intaglio printing method, and relief printing method.

Subsequently, the barium layer 6 a and the aluminum layer 6 b are formedon the exposed surface of the light-emitting layer 5 by a vacuum vapordeposition method. Thus, the cathode 6 is formed.

Although not illustrated in FIG. 1, the organic EL element 1 mayadditionally have a sealing layer on the surface of the cathode 6 or asealing cap to isolate the entire organic EL element 1 from externalspace. Such a sealing layer or sealing cap prevents the organic ELelement 1 after the completion from being exposed to the atmosphere. Thesealing layer may be formed, for instance, of materials such as SiN(silicon nitride) and SiON (silicon oxynitride), and may be disposed toencapsulate the organic EL element 1 therein. When a sealing cap isadditionally provided, the sealing cap may be formed of, for instance,the same material as the substrate 7, and a getter which absorbsmoisture and the like may be provided within the space enclosed by thesealing cap.

Through the manufacturing steps described above, the organic EL element1 is completed.

(Effects Achieved by Manufacturing Method of Organic EL Element)

The above-described manufacturing method of the organic EL element 1involves the UV irradiation step of radiating ultraviolet light havingthe predetermined wavelength, performed after the hole injection layer 3of tungsten oxide is formed. With the step, adsorbates on the surface ofthe hole injection layer 3 are removed, while the energy level resultingfrom structures similar to oxygen vacancies of metal oxide is maintainedon the hole injection layer surface.

Furthermore, the energy level mentioned above is continuously maintainedin the atmosphere throughout the time from the cleaning of the holeinjection layer 3 to the formation of the buffer layer 4. Consequently,the ability of hole injection is maintained with stability. This ensuresthe stable manufacturing of the organic EL element 1 that is driven withlow drive voltage and has longevity.

Furthermore, the duration of UV irradiation and the intensity ofultraviolet light in the UV irradiation step are determined in view ofthe conditions with which, with respect to a photoelectron spectrumexhibited by the hole injection layer 3, changes in shape of a spectralregion corresponding to a predetermined binding energy range converge.That is, the irradiation conditions are determined to achieve themaximum removable of adsorbates with the minimum conditions. Thus, thehighly stable hole injection efficiency is realized with a minimumcleaning process.

<Experiments and Observations>

(Effect of Adsorbates Removal by UV Irradiation)

According to this embodiment, subsequently to its formation, the holeinjection layer 3 composed of tungsten oxide is exposed to ultravioletlight under the predetermined conditions, whereby adsorbates are removedfrom the surface of the hole injection layer 3. The adsorbate removaleffect achieved by the UV irradiation is confirmed by the followingexperiments.

By the manufacturing method according to the present embodiment, sampleswere prepared each by laminating the anode 2 composed of ITO on thesubstrate 7, and the hole injection layer 3 composed of tungsten oxideon the anode 3, in the chamber of the sputtering film-forming apparatus.After the lamination, each intermediate sample was taken out of thechamber to the atmosphere to prepare samples without any UV irradiation,samples with UV irradiation for one minute, and samples with UVirradiation for ten minutes. The irradiation intensity was 155 mW/cm².

In the following description in the present embodiment, a sample withoutUV irradiation may be referred to as a “no-irradiation sample” and asample with UV irradiation for n minutes may be referred to as an“n-minute irradiation sample”.

Each sample was then attached to a photoelectron spectroscopy apparatus(PHI 5000 VersaProbe) manufactured by ULVAC-PHI, Incorporated to measurethe X-ray photoelectron spectroscopy (XPS). Generally, an XPS spectrumindicates the elemental composition, binding condition, and valence ofthe target surface up to several nanometers in depth. That is, ifelements not originally contained in tungsten oxide are observed, it ishighly likely that the elements are adsorbates. In addition, it isgenerally known that molecules adhere as a result of atmosphericexposure or during a manufacturing process are mainly carbon-containingmolecules, if water molecules and oxygen molecules are excluded.Therefore, the adsorbates removal effect achieved is confirmed bymeasuring changes in the carbon concentration in the surface region ofthe hole injection layer 3.

The conditions under which the XPS measurement was conducted are asfollows. Note that no charge-up occurred during the measurement.

Light source: Al K α

Bias: None

Electron emission angle: Normal line direction to the substrate surface

First, each sample was subjected to wide-scan measurement. As a resultof the measurement, the only elements found in each sample were tungsten(W), oxygen (O), and carbon (C). Then, narrow-scan spectra of eachsample were measured for the W 4f orbital (W4f) and also for the C1sorbital (C1s) to obtain the relative value of the number density ofcarbon atoms to the number of density of tungsten atoms present in thesurface region up to several nanometers in depth of the hole injectionlayer 3 composed of tungsten oxide. That is, the composition ratiobetween W and C was obtained. The composition ratio was obtained fromthe spectra, by using the composition ratio calculation function of“MultiPak” XPS, which is analyzing software included with thephotoelectron spectroscopy apparatus used in the measurements.

Table 1 below shows the composition ratio between W and C of eachsample.

TABLE 1 Composition Ratio between Sample Name W and C (W:C)No-Irradiation Samples 1:1.27 1-Minute Irradiation Samples 1:0.8310-Minute Irradiation Samples 1:0.62

With reference to Table 1, the number of carbon atoms relative to thenumber of tungsten atoms decrease more and more as the irradiationduration is longer, which is apparent when comparing the samples withoutirradiation against the samples with 1-minute irradiation and thesamples with 10-minute irradiation. That is, it is made clear that theUV irradiation according to the present embodiment serves to decreaseadsorbates on the surface of the hole injection layer 3 composed oftungsten oxide.

(Influences of UV Irradiation on Hole Injection Ability)

According to the present embodiment, the UV irradiation is performed toremove adsorbates from the surface of the hole injection layer 3composed of tungsten oxide, in a manner that the energy level resultingfrom structures similar to oxygen vacancies is maintained without anysubstantial influence. Note that the structures similar to oxygenvacancies favorably affect the hole injection ability. This property ofmaintaining structures similar to oxygen vacancies is confirmed by thefollowing experiments.

In the experiments, the above-described samples without irradiation,with 1-minute irradiation, and with 10-minute irradiation were subjectedto UPS (ultraviolet photoelectron spectroscopy) measurements. Generally,a UPS spectrum indicates the electronic state, from the valence band tothe Fermi surface (Fermi level), of the measurement target surface of upto several nanometers in depth. Especially in the case where themeasurement target is tungsten oxide or molybdenum oxide, the presenceof structures similar to oxygen vacancies on the layer surface isindicated by a protrusion appearing, in a UPS spectrum, near the Fermisurface in the low binding energy direction from the top of the valenceband (hereinafter, such a protrusion is referred to as a “spectralprotrusion near the Fermi surface”) (Non-Patent Literature 2 and 3).That is, by observing changes in the spectral protrusion near the Fermisurface before and after UV irradiation, the influence imposed by the UVirradiation on the structures similar to oxygen vacancies on the layersurface is examined. In tungsten oxide, the spectral protrusion near theFermi surface occupies a binding energy range that is from 1.8 eV to 3.6eV lower than the top of the valence band (the lowest binding energywithin the valence band).

The conditions under which the UPS measurement was conducted are asfollows. Note that no charge-up occurred during the measurement.

Light source: He I

Bias: None

Electron emission angle: Normal line direction to the substrate surface

FIG. 5 shows the UPS spectra of the respective samples, focusing onportions near the Fermi surface. In the following description, everyphotoelectron spectroscopy spectrum (UPS and XPS) is shown with thehorizontal axis representing the binding energy having the origin pointin the Fermi surface and with the left direction relative to the originpoint being positive. In all the spectra measured on the samples withoutirradiation, with 1-minute irradiation, and with 10-minute irradiation,a spectral protrusion near the Fermi surface is clearly observed. In thefigure, spectral protrusions near the Fermi surface are collectivelydenoted by (I). These results indicate that the structures similar tooxygen vacancies favorably affecting the hole-injection ability aremaintained even after the UV irradiation.

For the purpose of comparison, UV ozone cleaning was also performed.More specifically, samples were prepared each by laminating the anode 2composed of ITO on the substrate 7, and the hole injection layer 3composed of tungsten oxide on the anode 3, in the chamber of asputtering film-forming apparatus. The intermediate samples were thantaken out of the chamber to the atmosphere, followed by UV ozonecleaning of the surface of the hole injection layer 3 by a UV ozoneapparatus. The samples after the UV ozone cleaning were subjected to UPSmeasurement to check the presence of a spectral protrusion near theFermi surface.

FIG. 6 illustrates the UPS spectrum measured on the hole injection layer3 composed of tungsten oxide having been subjected to the UV ozonecleaning for three minutes, focusing on a portion near the Fermisurface. For the purpose of comparison, FIG. 6 also illustrates the UPSspectrum of the samples without irradiation, which is illustrated inFIG. 5. Different from the results shown in FIG. 5 regarding the samplessubjected to the UV irradiation according to the present embodiment, nospectral protrusion near the Fermi surface is observed at all. Theresults indicate that through the UV ozone cleaning, almost allstructures similar to oxygen vacancies are lost from the surface of thehole injection layer 3.

As described above, it is clarified that cleaning by the UV irradiationaccording to the present embodiment is different from the UV ozonecleaning in that structures similar to oxygen vacancies are maintainedwithout being lost. That is, structures similar to oxygen vacancies,which favorably affect the hole injection ability, are not eliminated bythe UV irradiation.

(Regarding Method for Determining UV Irradiation Conditions)

According to the present embodiment, the surface of the hole injectionlayer 3 composed of tungsten oxide is cleaned by UV irradiation. It isconfirmed by the following experimental results that the adsorbateremoval effect becomes saturated with the irradiation for a specificduration or longer.

In the same manner as described above, samples without irradiation, with1-minute irradiation, and with 10-minute irradiation were prepared. Inaddition, samples with 60-minute irradiation and 120 minute irradiationwere prepared. Then, narrow-scan spectra for W4f and C1s of therespective samples were measured by XPS measurement. From the respectivespectra, background components are subtracted. Then, the photoelectronintensity is normalized using the intensities by the area intensity. Thenarrow-scan spectra for C1s of the respective samples are shown in FIG.7. The area intensity of each C1s spectrum illustrated in FIG. 7 isproportional to the ratio of the number density of carbon atoms to thenumber density of tungsten atoms, all of which were present in thesurface region of the hole injection layer 3 composed of tungsten oxideup to several nanometers in depth from the layer surface.

According to FIG. 7, the C1s spectra measured on the samples with1-minute irradiation or longer are all substantially equal in intensity.This indicates that the adsorbate removal effect has substantiallyreached a level of saturation with the irradiation for the duration ofone minute or longer.

Generally, a C1s spectrum tends to be low in intensity and roughlyirregular as shown in FIG. 7, because the amount of adsorbates isintrinsically small. Therefore, C1s spectra may not serve the best indetermining saturation of the adsorbate removal effect. In view of this,the following describes other methods involving the use of spectra ofrelatively strong intensity, for determining saturation of the adsorbateremoval effect.

The first of such a method is to make a saturation determination basedon changes in the shape of a UPS spectral region corresponding to arange of binding energy around the top of the valence band (i.e., theUPS spectral region corresponding to the binding energy range from 4.5eV to 5.4 eV). A peak or shoulder appearing in the spectral regionindicates a lone pair of electrons in the 2p orbital in oxygen atomsconstituting tungsten oxide.

FIG. 8 illustrates the UPS spectra. The UPS measurements were made onthe respective samples without irradiation, with 1-minute irradiation,and with 10-minute irradiation. The photoelectron intensity isnormalized using a gentle peak appearing around the binding energy of6.5 eV. As shown in FIG. 8, the spectra measured on samples with1-minute irradiation and with 10-minute irradiation both have a clearpeak (denoted by (ii) in the figure) appearing in the binding energyrange from 4.5 eV to 5.4 eV. Such a peak does not appear in the spectrummeasured on samples without irradiation. In addition, the respectivespectra measured on samples with 1-minute irradiation and with 10-minuteirradiation are substantially identical in the shape of the peak. Thismeans that changes in the UPS spectral shape within the binding energyrange from 4.5 eV to 5.4 eV substantially converge with the irradiationfor the duration of one minute or longer. This behavior is similar tothat observed in C1s spectra. In addition, this behavior is assumed toindicate, similarly to C1s spectra, that the adsorbate removal effect isobtained by UV irradiation and that the effect becomes saturated withthe irradiation performed for the duration of one minute or longer.

The second one of such a method uses XPS measurements to make asaturation determination based on changes in the W4f spectral shape.FIG. 9 shows W4f spectra measured on the respective samples withoutirradiation, with 1-minute irradiation, with 10-minute irradiation, with60-minute irradiation, and with 120-minute irradiation. The spectra arenormalized using the maximum and minimum values.

As shown in FIG. 9, all the samples with irradiation exhibit a peaksteeper than a peak exhibited by the samples without irradiation (i.e.,the half-width of each peak is smaller). It is noted, in addition, thatthe peak shape is slightly steeper for the samples with 10-minuteirradiation than for the samples with 1-minute irradiation. Yet, for thesamples with 10-minute irradiation, with 60-minute irradiation, and with120-minute irradiation, the spectra coincide substantially entirely.This means that changes in the spectral shape converge for any sampleswith irradiation performed for the duration of ten minutes or longer.

Such changes in shape of W4f spectra resulting from differentirradiation durations are explained in the following way, for example.Although it depends on the configuration of adsorbates, provided thatthe adsorbates supply negative charges to tungsten atoms present on thelayer surface, the binding energy of the inner-shell orbital W4f becomeslower according to the negative charges. Chemically speaking, some ofhexavalent tungsten atoms present on the layer surface of tungsten oxidechange into lower-valent atoms, such as pentavalent atoms. In the XPSspectrum for W4f, this energy level shift is observed as a broaderspectral shape because of the spectrum for hexavalent tungsten atoms,which make up the majority, overlaps with the spectrum for lower-valenttungsten atoms, which make up a small proportion.

In view of the above, with respect to the spectra illustrated in FIG. 9,it is assumed that the peak is sharper in shape because the removal ofadsorbates by the UV irradiation alters pentavalent tungsten atoms backinto hexavalent atoms. Form the above observation, it is understood thatmost of the adsorbates are removed by the UV irradiation performed forone minute and that the adsorbate removal effect has reached a level ofsaturation with the UV irradiation performed for ten minutes or longer.This behavior is similar to that observed on C1s.

In addition, although not illustrated in the figure, it is confirmedthat the changes in the shape of the spectra for O1s orbital of oxygenatoms converge with the UV irradiation performed for ten minutes orlonger.

From the above, the adsorbate removal effect achieved by the UVirradiation according to the present embodiment becomes saturated withthe UV irradiation performed for a certain duration or longer. In thecase where the metal oxide is tungsten oxide, the irradiation conditionsare determined as follows. For example, the irradiation duration isdetermined by measuring, with respect to any specific irradiationintensity, the time taken for changes in the shape of the narrow-scanspectrum for W4f or O1s in XPS measurement converge or changes in theshape of UPS spectral region corresponding to the binding energy rangefrom 4.5 eV to 5.4 eV converge. The time thus measured is determined tobe the irradiation duration. More specifically, a spectrum measuredafter the UV irradiation for n-minute is compared with a spectrummeasured after the UV irradiation for (n+1)-minute to obtain thedifference between the two spectra at each of a plurality of measurementpoints. If the root-mean-square of the differences in the normalizedintensity becomes equal to a specific value or smaller, it is thendetermined that the changes in the spectral shape converge with theirradiation duration of n-minutes and thus the maximum level ofadsorbate removal has been completed. In this embodiment, it isdetermined from FIGS. 8 and 9 that the adsorbate removal effect becomessaturated with the UV irradiation performed for ten minutes.

(Regarding Maintaining Electronic State after UV Irradiation)

According to the present embodiment, the energy level resulting fromstructures similar to oxygen vacancies, which favorably affect thehole-injection ability, is maintained throughout the time from thesurface cleaning and at least until another layer is formed on thecleaned surface. The grounds are as follows.

The UPS spectra shown in FIG. 5 described above were measured two daysafter the UV irradiation. That is, between the samples withoutirradiation and the samples with the respective irradiation durationsthat were left to stand in the atmosphere for two days after the UVirradiation, there is no notable difference in the spectral protrusionnear the Fermi surface of the UPS spectra. In each UPS spectrum, thespectral protrusion is clearly observed. In addition, although notillustrated in the figures, measurements were made on samples two hoursafter the UV irradiation and one day after the UV irradiation. In thesemeasurements, the spectral protrusion near the Fermi surface was clearlyobserved in each spectrum in a manner similar to FIG. 5. That is to say,it is confirmed that the energy level resulting from structures similarto oxygen vacancies are sustained in the atmosphere at least for twodays after the UV irradiation.

This time period of two days is sufficiently long as compared with thetime normally lapsed until the step of laminating the buffer layer 4 onthe hole injection layer 3 is completed after the step of cleaning thehole injection layer 3 by UV irradiation. That is, unless the step offorming the buffer layer 4 is intentionally delayed, it is unlikely thatthe buffer layer 4 is not formed until after this two-day period.

(Regarding Improvements on EL Element Characteristics by UV Irradiation)

The organic EL element 1 according to the present embodimentmanufactured using the step of cleaning the hole injection layer 3 by UVirradiation exhibits better characteristics as compared with an organicEL element manufactured without UV irradiation. Such characteristics areconfirmed by the following experiments.

Firstly, the inventors prepared hole-only devices as assessment devicesto be used in accurately determining the effect on the hole injectionefficiency achieved by removing adsorbates from the surface of the holeinjection layer 3 by UV irradiation.

Generally, in an organic EL element, electric current is formed ofcarriers, which consists of holes and electrons. As such, the electricalcharacteristics of an organic EL element reflects electron current aswell as hole current. However, since, in a hole-only device, theinjection of electrons from the cathode is blocked, there is almost noflow of electron current. Thus, electrical current flowing in ahole-only device consists almost entirely of hole current. In otherwords, it could be considered that only holes function as a carrier in ahole-only device. Thus, a hole-only device is ideal in making anassessment of hole injection efficiency.

In detail, the hole-only devices 1A prepared were actually obtained byreplacing the cathode 6 of the organic EL element 1 illustrated in FIG.1 with gold (Au) to form a cathode 6A as illustrated in FIG. 10.Specifically, by following the manufacturing method of the organic ELelement 1 according to the present embodiment, a 50 nm-thick ITO thinfilm is formed as the anode 2 on the substrate 7 by a sputtering method,as illustrated in FIG. 10. Then, a 30 nm-thick tungsten oxide layer isformed as the hole injection layer 3 on the anode 2, by a predeterminedsputtering method in a manner that the layer surface has the energylevel resulting from structures similar to oxygen vacancies. Then, a 20nm-thick layer of TFB, which is an amine-containing organic polymer, isformed as the buffer layer 4 on the hole injection layer 3, and a 70nm-thick layer of F8BT, which is an organic polymer, is formed as thelight-emitting layer 5. Finally, a 100 nm-thick layer of gold is formedas the cathode 6A on the light-emitting layer 5.

Note that two hole-only devices 1A were prepared. One of the hole-onlydevices 1A had the hole injection layer 3 exposed to the UV lightaccording to the present embodiment (for the irradiation duration of 10minutes) after the hole injection layer 3 is formed and taken out of thechamber of the sputtering film-forming apparatus. The other of thehole-only devices 1A had the hole injection layer 3 not exposed to UVlight. Hereinafter, the former hole-only device 1A is referred to as“HOD with irradiation”, whereas the latter hole-only device 1A isreferred to as “HOD without irradiation”.

Each of the hole-only devices 1A thus prepared was then connected to theDC voltage source 8, so that voltage was applied thereto. Further, thevoltage applied to each sample was changed to measure the values ofelectric current flowing at different voltages. Each current value isthen converted into a value per unit area (current density). Note thathereinafter, the “driving voltage” refers to a voltage applied to obtainthe current density value is 0.4 mA/cm².

The hole injection efficiency of the hole injection layer 3 is said tobe higher as the driving voltage is smaller, for the following reason.That is, the members composing the hole-only devices 1A, other than thehole injection layer 3, were prepared according to the samemanufacturing method. Thus, it could be assumed that the hole injectionbarrier between two adjacent layers, other than that between the holeinjection layer 3 and the buffer layer 4 is uniform in each of thehole-only devices 1A. Considering the above, it could be expected thatthe differences in driving voltage of the hole-only devices 1A resultingfrom whether or not the surface of the hole injection layer 3 wasexposed to UV light closely reflects the hole injection efficiency fromthe hole injection layer 3 to the buffer layer 4.

Table 2 illustrates each of the hole-only devices 1A and a drivingvoltage thereof.

TABLE 2 Sample Name Drive Voltage HOD with Irradiation 18.9 V HODwithout Irradiation 19.7 V

In addition, FIG. 11 illustrates an electric current density-appliedvoltage curve of each of the hole-only devices 1A. In the figure, thevertical axis indicates electric current density (mA/cm²), whereas thehorizontal axis indicates applied voltage (V).

As shown in Table 2 and FIG. 11, when comparing the HOD with irradiationto the HOD without irradiation, the drive voltage is lower and therising of the electric current density-applied voltage curve is quicker.Further, it could be seen that HOD with irradiation requires for thelowest level of applied voltage to reach a high electric current densitycompared with HOD without irradiation. That is, HOD with irradiation hasa higher degree of hole injection efficiency compared with HOD withoutirradiation.

In the above, observation has been made of the hole injection efficiencyof the hole injection layer 3 in each of the hole-only devices 1A.However, it should be emphasized that the hole-only devices 1A and theorganic EL element 1 illustrated in FIG. 1 have nearly the samestructure, differing only in the cathode 6A. That is, the organic ELelement 1 is essentially the same as hole-only devices 1A in terms ofthe effect imposed by the adsorbate removal by the UV irradiation on theefficiency of hole injection form the hole injection layer 3 to thebuffer layer 4.

In order as to confirm the above, two samples of organic EL element 1were prepared. One of the samples were prepared using the hole injectionlayer 3 exposed to UV light, and the other of the samples were preparedusing the hole injection layer 3 not exposed to UV light. Hereinafter,the former sample of the organic EL element 1 is referred to as “BPDwith irradiation”, whereas the latter is referred to as “BPD withoutirradiation”. Except that the hole injection layer 3 of the BPD withoutirradiation was not exposed to UV light, the BPDs were manufactured bythe manufacturing method according to the present embodiment.

Each sample organic EL apparatus 1 thus prepared was connected to the DCpower source 8, so that voltage was applied thereto. Further, thevoltage applied to each sample was changed to measure the values ofelectric current flowing at different voltages. Each current value isthen converted into a value per unit area (current density). Note thathereinafter, the “driving voltage” refers to a voltage applied to obtainthe current density value is 10 mA/cm².

Table 3 illustrates each of the sample organic EL elements 1 and adriving voltage thereof.

TABLE 3 Sample Name Drive Voltage BPD with Irradiation 8.3 V BPD withoutIrradiation 9.2 V

In addition, FIG. 12 illustrates an electric current density-appliedvoltage curve of each of the sample organic EL elements 1A. In thefigure, the vertical axis indicates electric current density (mA/cm²),whereas the horizontal axis indicates applied voltage (V).

As shown in Table 3 and FIG. 12, when comparing the BPD with irradiationto the BPD without irradiation, the drive voltage is lower and therising of the electric current density-applied voltage curve is quicker.Further, it could be seen that BPD with irradiation requires for thelowest level of applied voltage to reach a high electric current densitycompared with BPD without irradiation. This tendency is the same as thatobserved with the HOD with irradiation and HOD without irradiation.

By the above experiments, it is confirmed regarding the organic ELelement 1 that the effect imposed on the hole injection efficiency fromthe hole injection layer 3 to the buffer layer 4 as a result of removalof adsorbates by UV irradiation to the surface of the hole injectionlayer 3 is similar to that confirmed with the hole-only devices 1A.

By the above experiments, the following is confirmed. That is, by UVirradiation performed in a predetermined manner according to the presentembodiment after the hole injection layer 3 is formed, adsorbates areremoved to the maximum extent from the surface of the hole injectionlayer 3 without affecting the energy level resulting from structuressimilar to oxygen vacancies. This means that adsorbates, which arelikely to cause increase of the drive voltage and decrease of the lifeof the organic EL element 1, are removed without impairing the holeinjection ability. Consequently, the efficiency in injecting holes fromthe hole injection layer 3 to the buffer layer 4 is improved, so thatexcellent characteristics of the organic EL element is realized.

(Regarding Wavelength of Ultraviolet Light)

According to the present embodiment, after the hole injection layer 3 isformed, adsorbates on the hole injection layer 3 is removed by radiatingultraviolet light of a predetermined wavelength in the atmosphere. Anorganic EL element 1 having the hole injection layer 3 having beensubjected to the adsorbates removal operates on a lower drive voltagethan an organic EL element manufactured without removal of adsorbates.The wavelength of ultraviolet light was determined through the followingobservations.

First, the wavelength of ultraviolet light that generates ozone (O₃) ina gas atmosphere containing oxygen molecules (O₂), such as in theatmosphere is 184.9 nm. By the following reaction, the oxygen moleculesare decomposed by ultraviolet light at 184.9 nm to yield oxygenradicals, which are then combined with remaining oxygen molecule togenerate ozone.O₂→O+OO+O₂→O₃

In addition, the wavelength of ultraviolet light causing furtherdecomposition of ozone to yield oxygen radicals again is 253.7 nm.

In UV ozone cleaning, ultraviolet light at 184.9 nm and 253.7 nm isemployed to generate oxygen radicals and their strong oxidation is usedto remove adsorbates. Therefore, as confirmed by the experimentsdescribed above, there is a risk that the energy level resulting fromstructures similar to oxygen vacancies disappears almost completely fromthe layer surface by UV ozone cleaning.

In view of the above risk, the present embodiment uses ultraviolet lightin a wavelength region of 184.9 nm or longer as such ultraviolet lightis not likely to cause decomposition of oxygen molecules to yield oxygenradicals. It is also preferable to use ultraviolet light within awavelength region of 253.7 nm or longer in order to avoid decompositionof atmospheric oxygen into ozone to yield oxygen radicals although theamount of such oxygen is small.

The metal halide lamp actually used in the present embodiment has aspectral distribution illustrated in FIG. 13. As illustrated in thefigure, the present embodiment uses a lamp that emits light ofwavelengths of 253.7 nm or shorter as little as possible. In lightemitted by the metal halide lamp, the intensity at 253.7 nm or shorteris at most a few percent of the maximum intensity (at about 380 nm).

Next, the binding energies between different combinations of atoms thatmay present in typical adsorbates are shown in Table 4. In the table,the mark “═” indicates double bond, whereas the mark “—” indicatessingle bond. To remove adsorbates, first, it is required to irradiatethe layer surface with light having energy stronger than the bondingenergies to break the bonds.

TABLE 4 Binding Binding Energy C═C 607 C—C 348 C═O 724 C—O 352 C—H 413O═O 490 O—O 139 O—H 463

Note that the light energy E per mol of photons and the wavelength λ arein the inverse proportion shown below.E=Nhc/λ

(N: Avogadro's number, h: Planck's constant, c: velocity of light, andλ: wavelength)

From the above expression, the energy of ultraviolet light at thewavelength 184.9 nm is calculated to be 647 kJ/mol. Similarly, theenergy of ultraviolet light at the wavelength 253.7 nm is calculated tobe 472 kJ/mol. With reference to Table 4, the energy value of theultraviolet light in the wavelength region determined according to thepresent embodiment is sufficient to disconnect most of atomic bondstypically appearing in adsorbates. Especially, as will be laterdescribed in detail, in the case of chemical adsorption, adsorbatesmainly make single bonds to oxygen atoms present in tungsten oxide. Thestrongest singe bond with atoms present in adsorbates is O—H bond withthe bonding energy of 463 kJ/mol (corresponding to wavelength of 258 nm)or so. Therefore, the ultraviolet light within the wavelength region ofthe present embodiment is strong enough to break the chemical bond. Inthe case of physical adsorption, the bonding is far weaker than chemicaladsorption, so that such adsorbates are readily removed by UVirradiation.

The above describes the reason why the ultraviolet light used in thepresent embodiment is sufficient to remove adsorbates.

Here, in terms of the efficiency of adsorbate removal, UV ozone cleaningis essentially better than the UV radiation according to the presentembodiment. This is because the UV ozone cleaning ensures thatadsorbates being unbonded are immediately oxidized with oxygen radicalsto form molecules such as CO₂ and H₂O, which easily migrate. As has beenalready described, however, UV ozone cleaning is not suitable forcleaning the hole injection layer 3 composed of metal oxide, such astungsten oxide.

To be noted next is that atomic bonds occurring in metal oxide is notgenerally broken by the energy of ultraviolet light within thewavelength region according to the present embodiment. According toNon-Patent Literature 3, for example, the binding energy between oxygenatom and tungsten atom in tungsten oxide is 672 kJ/mol (corresponding tothe wavelength of 178 nm). That is, it is difficult to break the bondbetween oxygen atom and tungsten atom with ultraviolet light within thewavelength region according to the present embodiment. This is incontrast with the above-described sputter etching performed in vacuum byusing argon ion. That is, with the use of ultraviolet light according tothe present embodiment, adsorbates are removed without breaking atomicbonds present in the hole injection layer 3 composed of metal oxide,such as tungsten oxide. That is, adsorbates are removed without makingthe hole injection layer 3 chemically active, i.e., while the holeinjection layer 3 is chemically stable.

For the reasons described above, the present invention uses ultravioletlight at the wavelength of 184.9 nm or longer, preferably 253.7 nm orlonger. Note that visible light is generally incapable of breaking thebond of chemical adsorption. The present embodiment therefore usesultraviolet light (380 nm or shorter), rather than visible light.

(Reason for which Energy Level Affecting Hole Injection Ability isMaintained After UV Irradiation)

According to the present embodiment, the energy level resulting fromstructures similar to oxygen vacancies on the layer surface iscontinuously maintained even after UV irradiation and thus the holeinjection ability is maintained with stability. That is, the presentembodiment ensures manufacturing of organic EL elements which operate onlow drive voltage. This property of maintaining the energy level isconsidered below.

It has been frequently reported, with reference to results ofexperiments and first principles calculations, that existence of theenergy level which can be seen in a thin film of, or a crystal oftungsten oxide, derives from structures similar to oxygen vacancies.More specifically, it is assumed that the existence of the energy levelon interest results from bonding orbitals formed, by the absence ofoxygen atoms, from the 5d orbitals of adjacent tungsten atoms and alsoresults from the 5d orbitals of singular tungsten atoms not terminatedwith an oxygen atom and exist on the layer surface or within the layer.

Here, it may be assumed that 5d orbitals of tungsten atoms are morestable when present in chemically adsorbed adsorbates, as compared withthe case where the 5d orbitals are present as bonding orbitals or as the5d orbitals of singular tungsten atoms. However, such an assumption isnot necessarily correct. Actually, as observed in the UPS spectraillustrated in FIG. 5, tungsten oxide left to stand in the atmospherefor two day exhibits a spectral protrusion near the Fermi surface, whichis the indication of the energy level being discussed.

In Non-Patent Literature 4, it has been reported that when tungstentrioxide single crystal is cleaved in vacuum to expose the clean (001)surface, part of oxygen atoms present on the outermost surface areemitted. Non-Patent Literature 4 further reports the following reason,which has been confirmed by the first principles calculations, for whichthe (001) surface is more stable in terms of energy when a tungsten atomnot terminated with an oxygen atom is periodically present on theoutermost surface as shown in FIG. 14 than when all the tungsten atomsare terminated with an oxygen atom. That is, when all the tungsten atomspresent on the outermost surface are terminated with an oxygen atom, theelectrical repulsive force occurring between terminal oxygen atomsbecomes large, which causes the instability. In short, the (001) surfaceis more stable when structures similar to oxygen vacancies (a) arepresent on the surface.

FIG. 14 illustrates tungsten oxide atoms each surrounded by six oxygenatoms to form an octahedron with the six oxygen atoms at the vertices.For the simplicity sake, in the figure, the octahedrons are arranged inorderly succession in a manner similar to the rhenium oxide structure.In practice, however, the octahedrons are distorted to some extent andarranged without such orderliness.

By analogy with the above findings, the following mechanism may be oneexample of the reason for which the energy level resulting fromstructures similar to oxygen vacancies is maintained on the surface ofthe hole injection layer 3 continuously after the UV irradiationaccording to the present embodiment.

According to the present invention, the hole injection layer 3 composedof tungsten oxide is assumed to have a (001) facet at least locally onthe layer surface immediately after the formation of the hole injectionlayer 3. That is, as illustrated in FIG. 14, the hole injection layer 3is assumed to have terminal oxygen atoms (b) and tungsten atoms notterminated with an oxygen atom (a). Non-terminated tungsten atoms (a)are surrounded by terminal oxygen atoms (b). It is because the (001)surface has a stable structure. It is this surface that is exposed toimpurity molecules and atomic molecules in the chamber of the sputteringfilm-forming apparatus subsequently to the formation of the holeinjection layer 3.

In general, if unsaturated metal atoms such as (a) are present on thelayer surface of metal oxide, the metal atoms tend to be terminated witha water or organic molecule as a result of chemical adsorption. In thepresent embodiment, however, none of the W4f spectra illustrated in FIG.9 have a peak in a binding energy range from 31 eV to 33 eV at which apeak derived from the bonding between a tungsten atom and a carbon atomshould appear. Instead, each W4f spectra illustrated in FIG. 9 has apeak derived from the bonding between a tungsten atom and an oxygenatom. It is therefore highly likely that the atoms of adsorbed moleculesto which tungsten atoms (a) are chemically bonded is oxygen atoms.

However, for example, a tungsten atom (a) may chemically react with awater molecule to form a hydroxyl group or a tungsten atom (a) maychemically react with an organic molecule to be bonded to an oxygen atomcontained in the organic molecule. In such cases, a repulsive force isgenerated between an adsorbed oxygen atom, which generally is innegatively charged, and adjacent terminal oxygen atoms (a), which arealso negatively charged. In view of the above, it is expected thatadsorption of molecules to tungsten atoms (a) is relatively unlikely,for the same reason for which tungsten atoms (a) are unlikely to haveterminal oxygen atoms in vacuum.

On the other hand, terminal oxygen atoms (b) surrounding tungsten atoms(a) undergo addition reaction with water molecules and organic moleculesto cause chemical adsorption. Such chemical adsorption occurs relativelyeasily as there is substantially no factor inhibiting adsorption, suchas repulsive force. In some cases, the chemical adsorption to terminaloxygen atoms (b) may result in that organic molecules composed of a fewor more atoms are present in the immediate vicinity of tungsten atoms(a). Such organic molecules act as spatial barriers to against adsorbingmolecules. Therefore, adsorption of molecules to terminal oxygen atoms(b) is also expected to prevent adsorption of molecules to tungstenatoms (a).

From the above consideration, in the layer surface having: terminaloxygen atoms (b); and tungsten atoms (a) not terminated with an oxygenatom and surrounded by the terminal oxygen atoms (b) as illustrated inFIG. 14, the occurrence of chemical adsorption to the tungsten atoms (a)is less likely. Instead, impurity molecules and atomic molecules tend tochemically adhere to the terminal oxygen atoms (b) surrounding tungstenatoms (a). Note that the chemical adsorption occurring in this case is abond via a terminal oxygen atom and thus generally is a single bond.

In response to the UV irradiation according to the present embodiment,only molecules chemically bonded to oxygen atoms (b) are disconnectedand released. As a result, it is expected that the oxygen atoms (b)revert to terminal oxygen atoms as they were before the chemicaladsorption or react with water molecules to form hydroxyl groups, whichare stable and not easily disconnected by the UV irradiation accordingto the present embodiment.

To summarize the above, the hole injection layer 3 composed of tungstenoxide according to the present embodiment has, on the layer surface, thelocal structure as illustrated in FIG. 14. That is, tungsten atoms (a)not terminated with an oxygen atom are surrounded by terminal oxygenatoms (b). First of all, this structure per se has a characteristicwhich prevents adsorption of molecules. In addition, molecules adheredto terminal oxygen atoms (b) are released by UV irradiation, after whichhydroxyl groups mainly remain present on the layer surface. In thismanner, while adsorbates are removed by UV radiation performed after thelayer formation, the electronic state resulting from structures similarto oxygen vacancies (a) on the layer surface is maintained without beingaffected by the UV irradiation. The electron state thus maintainedpositively affects the hole injection ability.

<UV Irradiation Apparatus>

Next, the following describes an UV irradiation apparatus employed in anembodiment of the present invention. FIG. 4B illustrates an UVirradiation apparatus 20 according to one aspect of the presentinvention. The UV irradiation apparatus 20 is for irradiatingultraviolet light to an intermediate product 9 of the organic EL element1 and includes: a light source 21 that emits ultraviolet light mainlywithin the wavelength region longer than 184.9 nm and equal to 380 nm orshorter; a reflector 22 that reflects ultraviolet light emitted by thelight source 21 toward the intermediate product 9; a housing 23 thathouses the light source 21 and reflector 22 to hold them in place; and acontrol unit 24 that controls ON/OFF of the light source 21.

The intermediate product 9 has, for example, the anode 2 and the holeinjection layer 3 composed of metal oxide layered on the substrate 7 butthe buffer layer 4 has not been formed yet.

The light source 21 is, for example, a straight metal halide lampdisposed to longitudinally coincide with the widthwise direction of theintermediate product 9 being transferred. The light source 21 isoperated to emit light under the conditions suitable for effectivemanufacturing of organic EL elements capable of emitting light at highintensity and low electrical consumption. The conditions of UVirradiation, such as irradiation duration and irradiation intensity, aredetermined based on various factors, including the formation conditionsof the hole injection layer 3, such as the type of metal oxide used, andthe convergence of changes in shape of photoelectron spectroscopyspectra exhibited by the samples of the hole injection layer 3 asdescribed in the present embodiment. The irradiation conditions are setby the operator. Alternatively, the irradiation conditions may beautomatically set by the control unit 24. For example, the control unit24 stores a database in which layer forming conditions, irradiationdurations, and irradiation intensities are correlated. On receivinginput of the layer forming conditions by the operator, the control unit24 sets the irradiation duration and intensity with reference to thedatabase.

The intermediate product 9 is transported to the position for UVirradiation by a conveyer 25. In the figure, the intermediate product 9placed onto the conveyer 25 from the upstream (right-hand side of thefigure) in the transport direction is transported by the conveyer 25 topass the position for receiving UV irradiation. While passing theposition, a predetermined amount of ultraviolet light is applied to theupper surface of the intermediate product 9, i.e., the upper surface ofthe hole injection layer 3. Having been irradiated with ultravioletlight, the intermediate product 9 is transported out to the downstream(left-hand side of the figure).

In the UV irradiation apparatus 20 described above, the light source 21is not limited to a metal halide lamp. Alternately, the light source 21may be any light source capable of emitting ultraviolet light mainlywithin the wavelength region longer than 184.9 nm and equal to 380 nm orshorter (preferably, longer than 253.7 nm and equal to 380 nm orshorter).

<Modifications>

Up to this point, a manufacturing method of an organic EL elementaccording to one aspect of the present invention has been specificallydescribed. In addition, an UV irradiation apparatus according to oneaspect of the present invention has been specifically described.However, the specific embodiment(s) described above is an example usedin order to clearly illustrate a structure of the present invention andthe effects and advantages thereof. The present invention is not limitedto the specific embodiment described above. For example, the sizesand/or materials specifically mentioned are merely typical examples usedto make it easier to understand the present invention. The presentinvention is not limited to such a specific size and/or material.

For example, the metal oxide contained in the hole injection layer 3 isnot limited to tungsten oxide. Alternatively, the hole injection layer 3may contain any one of, or a combination of any two or more of thefollowing metal oxides, namely, molybdenum oxide, chromium oxide,vanadium oxide, niobium oxide, tantalum oxide, titanium oxide, zirconiumoxide, hafnium oxide, scandium oxide, yttrium oxide, thorium oxide,manganese oxide, iron oxide, ruthenium oxide, osmium oxide, cobaltoxide, nickel oxide, copper oxide, zinc oxide, cadmium oxide, aluminumoxide, gallium oxide, indium zinc oxide, silicon oxide, germanium oxide,stannous oxide, lead oxide, antimony oxide, bismuth oxide, and an oxideof any of so-called rare-earth elements ranging from lanthanum tolutetium.

As an example of the hole injection layer composed of a metal oxideother than tungsten oxide, the hole injection layer composed ofmolybdenum oxide is described below. It is clarified by the followingexperiments that adsorbates on the surface of the hole injection layer 3decrease as a result of the cleaning by UV irradiation according to thepresent embodiment even if the hole injection layer 3 is composed ofmolybdenum oxide.

In the experiments, samples without irradiation, samples with 10-minuteirradiation, and samples with 60-minute irradiation were prepared in amanner similarly to the experiments on tungsten oxide.

First, each sample was subjected to XPS measurements to obtainnarrow-scan spectra for the Mo 3f orbital (Mo3d) and also for the C1sorbital (C1s). After appropriately subtracting background componentsfrom the respective spectra, the photoelectron intensity of thenarrow-scan spectra for Mo3d was normalized using the area intensity.The narrow-scan spectra for C1s of the respective samples are shown inFIG. 15. The area intensity of each C1s spectrum shown in FIG. 15 isproportional to the ratio of the number density of carbon atoms to thenumber density of molybdenum atoms present in the surface region of thehole injection layer 3 composed of molybdenum oxide. The surface regionis up to several nanometers in depth from the layer surface.

As shown in FIG. 15, each of samples with 10-minute irradiation and with60-minute irradiation exhibited C1s spectra with a weaker area intensityas compared with the sample without irradiation. From this observation,it is assumed that carbon atoms decrease by UV irradiation. In otherwords, adsorbates were removed by the UV irradiation.

Next, evaluations were made on the respective UPS spectra to see changesin the shape of UPS spectral regions each corresponding to the bindingenergy range around the top of the valence band (i.e., the UPS spectralregions each corresponding to the binding energy range from 3.7 eV to5.2 eV). A peak or shoulder appearing in this spectral region indicatesa lone pair of electrons in the 2p orbital in oxygen atoms constitutingmolybdenum oxide.

The UPS measurements were made on the respective samples withoutirradiation, with 10-minute irradiation, and with 60-minute irradiation.FIG. 16 illustrates the UPS spectra. The photoelectron intensity isnormalized using the intensity at the binding energy 6.2 eV.

As shown in FIG. 16, the spectrum of each of the samples with 10-minuteirradiation and with 60-minute irradiation exhibited a broad shoulderappearing in the region corresponding to the binding energy range from3.7 eV to 5.2 eV (this region is denoted by “(III)” in the figure), ascompared with the sample without irradiation. In addition, the shouldersof the spectra closely match in shape between the samples with 10-minuteirradiation and with 60-minute irradiation. This means that the changesin shape of UPS spectral portion within the range of binding energy from3.7 eV to 5.2 eV substantially converge with respect to the samples withirradiation duration of ten minute or longer. This is assumed toindicate that the adsorbate removal effect becomes saturated.

Next, evaluations were made on changes caused by the UV irradiation inthe shape of Mo3d spectra obtained by XPS measurement. FIG. 17 shows theMo3d spectra of the respective samples without irradiation, with10-minute irradiation, and with 60-minute irradiation. The spectra arenormalized using the maximum and minimum values.

As shown in FIG. 17, all the samples with irradiation exhibited a peakbroader than a peak exhibited by the samples without irradiation (i.e.,the half-width of the peaks is wider). In addition, the extent to whichthe half-width of the peak becomes wider is smaller as the irradiationduration is longer. This indicates that changes in spectral shape tendto converge as the irradiation duration is longer.

From the above, the following is also known. That is, even in the casewhere the metal oxide constituting the hole injection layer 3 ismolybdenum oxide, the adsorbate removal effect achieved by the UVirradiation according to the present embodiment becomes saturated at acertain irradiation duration or longer. In the case where the metaloxide is molybdenum oxide, the irradiation conditions are determined asfollows. For example, the irradiation duration is determined bymeasuring, with respect to any specific irradiation intensity, the timetaken for changes in the shape of the narrow-scan spectrum for Mo3d orO1s in XPS measurement converge or changes in the shape of UPS spectralregion corresponding to the binding energy range from 3.7 eV to 5.2 eVconverge. The time thus measured is determined to be the irradiationduration. More specifically, a spectrum measured after irradiation forn-minute is compared with a spectrum measured after irradiation for(n+1) minutes to obtain the difference between the two spectra at eachof a plurality of measurement points. If the root-mean-square ofdifferences in the normalized intensity becomes equal to a specificvalue or smaller, it is then determined that the change in the shape ofspectra converge as a result of the irradiation for the n-minuteduration and thus adsorbate removal at the maximum level has beencompleted. In this embodiment, it is determined from FIG. 16 that theadsorbate removal effect becomes saturated as a result of the UVirradiation for ten minutes.

(Additional Matters)

In the manufacturing method for organic EL elements according to oneaspect of the present invention, the UV irradiation is performed in theambient atmosphere. Alternatively, however, the UV irradiation may beperformed in various other gas atmospheres, such as reduced-pressureatmosphere, inert gas atmosphere, or vacuum. The above variations arepossible because the cleaning by UV irradiation uses ultraviolet lightat such wavelengths not generating oxygen radicals. Still, however, theUV irradiation performed in the atmosphere is advantages in themanufacture of large-sized panels, for the reasons stated above.

Further, the implementation of the organic EL element of the presentinvention is not limited to a structure where the organic EL element isused alone. A plurality of organic EL elements of the present inventionmay be integrated on a substrate as pixels to form an organic EL panel.An organic EL display so yielded may be implemented by appropriatelyarranging the thickness of each of the layers in each of the organic ELelements.

In the manufacture of organic EL panels using application-type organicEL elements, the step of integrating a plurality of organic EL elementsas pixels on a substrate is performed in the following manner, forexample. That is, banks defining the pixels are formed on the holeinjection layer composed of metal oxide and functional layers areoverlaid within the regions defined by the banks. The step of formingthe banks is performed in the following manner, for example. First, abank material composed of photosensitive resist material is applied ontothe surface of the hole injection layer, followed by pre-baking of thebank material. Then, the bank material is exposed to light via a patternmask to remove unhardened, redundant bank material with a developer,followed by rinsing with pure water. The present invention is applicableto the hole injection layer composed of metal oxide having undergone thebank forming step as above. In this case, by performing the UVirradiation of the surface of the hole injection layer after the banksare formed, organic molecules, which are residues of banks anddeveloper, are removed from the surface of the hole injection layer. Ingeneral, irradiating banks with ultraviolet light results in changes inthe contact angle of each bank with respect to an organic solventapplied as an upper layer. Yet, according to the present invention, itis easy to uniformly determine the irradiation conditions of ultravioletlight. Therefore, the contact angle and the bank configuration can beappropriately adjusted in view of the uniformly determined irradiationconditions.

The organic EL element according to one aspect of the present inventionmay be a so-called bottom emission type or a top emission type.

[Industrial Applicability]

The organic EL element pertaining to the present invention is to be usedas display elements for mobile phone displays and TVs, and as a lightsource for various applications. Regardless of the specific use thereof,the organic EL element of the present invention is applicable as anorganic EL element having a wide range of luminous intensity from lowluminous intensity to high luminous intensity for the use as a lightsource or the like, and which can be driven at a low voltage. Theorganic EL element of the present invention, for having such a highlevel of performance, may be used in a wide range of applications,including those for household use, those for use in public facilities,and those for professional use. More specifically, such applicationsinclude: various display devices; TV apparatuses; displays for portableelectronic devices; illumination light sources, and etc.

[Reference Signs List]  1 organic EL element  1A hole-only element  2anode  3 hole injection layer  4 buffer layer (functional layer)  5light-emitting layer (functional layer)  6 cathode  6a barium layer  6baluminum layer  6A cathode (Au layer)  7 substrate  8 DC voltage source 9 intermediate product  20 ultraviolet irradiation apparatus  21 lightsource  22 reflector  23 housing  24 control unit  25 conveyor 100display apparatus 200 light-emitting apparatus

The invention claimed is:
 1. A method of manufacturing an organic ELelement comprising: a first step of forming, on an anode, a holeinjection layer including metal oxide; a second step of irradiating thehole injection layer with ultraviolet light, the ultraviolet lighthaving a wavelength greater than a wavelength at which oxygen moleculesdecompose and yield oxygen radicals; a third step of forming functionallayers containing organic material on or above the hole injection layerafter the second step, the functional layers including a light-emittinglayer; and a fourth step of forming a cathode on or above the functionallayers.
 2. The method of claim 1, wherein in the second step, theultraviolet light has a wavelength greater than a wavelength at whichozone decomposes and yields oxygen radicals.
 3. The method of claim 1,wherein in the second step, the ultraviolet light has a wavelengthlonger than 184.9 nm and not longer than 380 nm as a main range.
 4. Themethod of claim 1, wherein in the second step, the ultraviolet light hasa wavelength longer than 253.7 nm and not longer than 380 nm as a mainrange.
 5. The method of claim 1, wherein the first step is performed invacuum, and the second step is performed in the atmosphere.
 6. Themethod of claim 1, wherein in the second step, the hole injection layeris irradiated with the ultraviolet light until a narrow-scan spectrumfor an inner-shell orbital of an atom included as a main element in themetal oxide stabilizes in XPS measurement.
 7. The method of claim 1,wherein in the first step, the metal oxide is tungsten oxide.
 8. Themethod of claim 7, wherein in the second step, the ultraviolet light hasa wavelength corresponding to an energy value equal to or greater thanbinding energy of a single bond between an oxygen atom in tungsten oxideand adsorbates to the oxygen atom, and smaller than binding energybetween an oxygen atom and a tungsten atom in tungsten oxide.
 9. Themethod of claim 8, wherein the adsorbates include at least one of carbonatom, hydrogen atom, oxygen atom and nitrogen atom.
 10. The method ofclaim 1, wherein the metal oxide is tungsten oxide, and in the secondstep, the hole injection layer is irradiated with the ultraviolet lightuntil UPS spectrum stabilizes within the binding energy range from 4.5eV to 5.4 eV.
 11. The method of claim 1, wherein the metal oxide ismolybdenum oxide, and in the second step, the hole injection layer isirradiated with the ultraviolet light until UPS spectrum stabilizeswithin the binding energy range from 3.7 eV to 5.2 eV.
 12. The method ofclaim 1, wherein in the second step, the hole injection layer isirradiated with the ultraviolet light until a narrow-scan spectrum forC1s of the hole injection layer stabilizes in XPS measurement.
 13. Themethod of claim 1, wherein the metal oxide is tungsten oxide, and in thesecond step, the hole injection layer is irradiated with the ultravioletlight until a narrow-scan spectrum for W4f of the hole injection layerstabilizes in XPS measurement.
 14. The method of claim 1, wherein themetal oxide is molybdenum oxide, and in the second step, the holeinjection layer is irradiated with the ultraviolet light until anarrow-scan spectrum for Mo3d of the hole injection layer stabilizes inXPS measurement.
 15. A display apparatus using an organic EL elementmanufactured by the method of claim
 1. 16. A light-emitting apparatususing an organic EL element manufactured by the method of claim
 1. 17.An ultraviolet irradiation apparatus irradiating ultraviolet light on anintermediate product of an organic EL element having a hole injectionlayer and functional layers layered between an anode and a cathode, thehole injection layer including metal oxide, the functional layersincluding organic material and having holes injected thereto from thehole injection layer, wherein the ultraviolet light has a wavelengthgreater than a wavelength at which oxygen molecules decompose and yieldoxygen radicals.
 18. The ultraviolet irradiation apparatus of claim 17,wherein the ultraviolet light has a wavelength greater than a wavelengthat which ozone decomposes and yields oxygen radicals.
 19. Theultraviolet irradiation apparatus of claim 17, wherein the ultravioletlight has a wavelength longer than 184.9 nm and not longer than 380 nmas a main range.
 20. The ultraviolet irradiation apparatus of claim 17,wherein the ultraviolet light has a wavelength longer than 253.7 nm andnot longer than 380 nm as a main range.